1. Field of the Invention
The present invention relates to a signal processor for reproducing an impulse response representing a transmission characteristic between two points of a transmitting system calculated in advance by measurement or calculation and a headphone device and a speaker device for which the signal processor is used.
2. Description of the Related Art
Conventionally, there is proposed an out-of-head audio image localizing headphone device in which an audio image formed by respective input audio signals from a left front sound source LF, a right front sound source RF, a left back sound source LB, and a right back sound source RB with respect to a listener is localized on an outside of the listener head.
FIG. 6 shows an arrangement of a prior-art out-of-head audio image localizing headphone device. In FIG. 6, the out-of-head audio image localizing headphone device is formed of input terminals 101 to 104 to which four-channel audio signals of the above left front sound source LF, right front sound source RF, left back sound source LB, and right back sound source RB are supplied, A/D (Analog-Digital) converters (AD1 to AD4) 105 to 108 for converting input audio signals into digital signals, a signal processor 109 for applying predetermined signal processing to the input audio signals converted into the digital signals, D/A (Digital-Analog) converters (DA1 and DA2) 110 and 111 for respectively converting outputs of the signal processor 109 into analog signals, amplifiers (AM1 and AM2) 112 and 113 for respectively amplifying the audio signals converted into the analog signals, and a headphone 114 to which the amplified audio signals are respectively supplied.
FIG. 7 is a block diagram showing an arrangement of a prior-art signal processor. In FIG. 7, the signal processor 109 is formed of digital FIR filters (F1 to F8) 125 to 132 for convoluting respective impulse responses from the above four-channel sound sources to a left ear and a right ear of the listener calculated in advance by measurement or calculation to input audio signals DLF, DRF, DLB, and DRB converted into the digital signals and supplied to input terminals 121 to 124, a pair of adding circuits 133 to 138 for respectively adding outputs of left channels and right channels of the digital FIR filters (F1 to F8) 125 to 132, and output terminals 139 and 140 for outputting outputs of the adding circuits 133 to 138 as output signals.
By the above prior-art headphone device, the same sound field as that in a case of listening to a reproduced sound obtained by reproducing four-channel input audio signals by a speaker was intended to be generated on an outside of a head of the listener.
However, according to the above-described prior-art headphone device, if the same sound field as that in the case of listening to a reproduced sound obtained by reproducing by a speaker was intended to be generated on an outside of a head of the listener, it was necessary to enormously increase a length of the impulse response convoluted by each of the digital FIR filters within the headphone device. Therefore, if each of the digital FIR filters was formed by using a digital signal processing IC (integrated circuit), the digital signal processing IC mounted with a high-speed product-sum operating circuit and a large number of memories was necessary. Thus, a size of the circuit increased and the system became extremely expensive.
It is an object of the invention to provide a signal processor in which the above problems are solved, a size of the circuit is reduced, an arrangement is simplified, a cost is reduced, and the same sound field as that in a case of listening to a reproduced sound obtained by reproducing by a speaker can be generated on an outside of a head of a listener and to provide a headphone device and a speaker device for which the signal processor is used.
Specifically, it is an object to provide a digital signal processor in which FIR filters for convoluting impulse responses are formed and which can perform convolution of the impulse responses having approximate lengths and characteristics to those of actual impulse responses by using the FIR filters with relatively short tap lengths.
A signal processor of the present invention is applied to a signal processor for carrying out predetermined signal processing by signal processing means for convoluting impulse responses from sound sources to a measurement point calculated in advance by measurement or calculation to input audio signals of a plurality of channels and outputting the signals.
The signal processing means divides the input audio signals into two lines for respective channels and has finite-tap-length FIR filters respectively for the plurality of channels and reverberation adding circuits respectively for the first line and the second line. At an addition point provided to a tap midpoint of the FIR filter of one channel of the first and second lines, a delay output signal of a tap end from the FIR filter of the other channel of the first and second lines is multiplied by a predetermined multiplication coefficient and then added. Output of the FIR filter of the one channel of the first and second lines is subjected to reverberation adding processing by the reverberation adding circuits of the first and second lines and then output of the FIR filter of the other channel of the first and second lines is added to the output of the FIR filter of the one channel to obtain addition output as an output signal of the first and second lines of the signal processing means.
A headphone device of the invention is a headphone device to which the above signal processor is applied, the headphone device driving a headphone by an output left audio signal and an output right audio signal to localize an audio image by input audio signals on an outside of a head.
A speaker device of the invention is a speaker device to which the above signal processor is applied, the speaker device driving the two speakers by the output left audio signal and the output right audio signal to localize an audio image by the predetermined audio signals in a position other than the two speakers.
The invention has the following operations.
By properly choosing a position of the tap of the digital FIR filter of the one channel of the first and second lines into which the delay output signal multiplied by the predetermined multiplication coefficient is input, a length of impulse response reproduced by the digital FIR filter of the one channel of the first and second lines, the digital FIR filter of the other channel of the first and second lines, and the reverberation adding circuits can be increased, even if a length of the impulse response convoluted in the digital FIR filter of the other channel of the first and second lines is smaller than a length of the impulse response convoluted in the digital FIR filter of the one channel of the first and second lines.
A main portion representing a characteristic of the impulse response to be reproduced is convoluted by the digital FIR filter of the other channel of the first and second lines and the output of the digital FIR filter of the other channel is added to the reverberation output of the reverberation adding circuit with respect to the digital FIR filter of the one channel of the first and second lines. Therefore, even if a characteristic of a signal component of a portion to be processed of the delay signal output from the tap end of the digital FIR filter of the other channel of the first and second lines is different from the characteristic of the impulse response to be reproduced, a reproduced characteristic of the impulse response can be substantially the same as the desired characteristic of the impulse response.
As a result, the same sound field as that in a case of listening to a sound obtained by reproducing the above input audio signals by speakers disposed in positions of the sound sources can be generated on an outside of a head of the listener.